A coin cell provides an attractive, low-cost and volume-efficient energy source solution for a firing circuit for a miniature electro-explosive device (EED). In this application, the ability to fire the EED reliably after a long shelf-life period measured in years, with a high degree of reliability, is desired. At the end of this shelf-life period, the coin cell is required to charge a capacitor quickly, resulting in a high pulse current load on the coin cell. A coin cell chemistry such as lithium poly carbon monofluoride [Li(CF)n] is well-suited in many respects for this application since it has a long shelf-life with a very low internal self-discharge rate. However, in applications such as this, the coin cell also powers the control circuitry which generally requires that the voltage remain above a threshold voltage for proper operation during this pulse loading of the coin cell. The coin cell output voltage is a function of the pulse current load and the internal impedance of the coin cell. Not much information exists in published literature on the effect of shelf life on the internal impedance of these coin cells, but testing after exposure to high temperature diurnal cycling (−38 C. to 70 C.) for a period of 30 days showed some degradation in the internal impedance.
Because of volume constraints, simply using a larger coin cell with a higher discharge rate capability (and thus a lower internal impedance) is not a feasible solution in this application. In addition, it is desired that the solution should be low-cost.
What is needed, therefore, are techniques to accommodate this degradation and extend the shelf-life capability of the coin cell.